Photoremovable Groups

More strictly connected with organic synthesis is photoinduced deprotection [251]. The most largely used photoremovable groups are based on the o-nitrobenzyl chromophore [22]. Intramolecular hydrogen abstraction leads to an acetal derivative, which under the reaction conditions collapses to o-nitrosobenzaldehyde and the liberated substrate (Sch. 25). 

Scheme 3 Structures of Photoremovable Groups Nvoc and a-Methyl-Substituted Npod l...

Photochemical triggers irreversibly release free target molecules and also photoproduct(s) formed by the transformation of photoremovable groups (Scheme 6.189). Their design must fulfil several requirements for example, the side-products should be chemically and photochemically stable and nontoxic (in biological applications) and the photoremovable moiety should absorb at wavelengths... 

Scheme 6.192 shows the o-nitrobenzyl moiety used as a photolabile linker in the solid-phase oligosaccharide synthesis on a polystyrene (PS) support.1208 After the synthesis of the protected oligosaccharide 404, PS attached to the photoremovable group is removed by photolysis and the final product is obtained by hydrogenolysis. Such a strategy could be promising for combinatorial synthesis of oligosaccharide libraries. 

A new method has been described for the protection of amino acids with a photoremovable group. This procedure involves the conversion of the amino acid into the phenacyl derivative (114). Irradiation of (114) in a buffered aqueous... 

Fig. 1 Caging fragrance molecules to a photoremovable protecting group (a) and releasing the fragrance molecules by exposure to sunlight (b).

The most extensively used photoremovable protecting groups are based on o-nitrotoluene derivatives, which, on exposure to light, undergo intramolecular H-atom abstraction to form photoenols (Scheme 9), followed by the elimination of their protected or caged substrate. ... 

PHOTOREMOVABLE PROTECTING GROUPS BASED ON PHOTOENOLIZATION 49 Table 1 Quantum yields for photoelimination from 18... 

In 2003, Banerjee et al. designed an efficient photoremovable protecting group for the release of carboxylic acids based on similar p-elimination from photoenols (Scheme 14). They showed that o-alkyl acetophenone derivatives with various ester groups in the p-position release their ester moiety in high chemical yields. The authors proposed that the photorelease took place as shown in Scheme 14 but did not support the mechanism with transient spectroscopy. Formation of 21, which is expected to be the major product in the reaction, was not confirmed, and thus, the authors speculated that 21 undergoes polymerization to yield oligomers. 

Scaiano and coworkers designed the two new photoremovable protecting groups 27 and 28, based on ketoprofen 25. Photolysis of 27 in aqueous solutions releases ibuprofen in quantum yields of 0.75 (Scheme 19). Furthermore, the rate constant for the release of ibuprofen from 27 was measured to be 5 x 10 s Thus, 27 can release ibuprofen both rapidly and in high quantum yields. 

Recently, Falvey and coworkers have designed photoremovable protecting group... 

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